When a regular polygon with \( n \) sides is decomposed into triangles using the same vertex, \( n-2 \) triangles are formed.
This is because you can choose a vertex of the polygon and connect it to every other non-adjacent vertex. For a polygon with \( n \) sides, you can form \( n-2 \) triangles.
Thus, the correct response is:
"When a regular polygon is decomposed, \( n-2 \) triangles are formed, where \( n \) is the number of sides of the regular polygon. There are two fewer triangles than the number of the sides of the regular polygon."